1. Field of the Invention
The present invention relates to a plasma processing apparatus for use with a plasma etching apparatus, a plasma CVD (chemical vapor deposition) apparatus, a plasma ashing apparatus or the like used in various semiconductor manufacturing processes, for example.
2. Description of the Prior Art
As shown in a cross-sectional view forming FIG. 1A, for example, when semiconductor integrated circuit devices are manufactured, it is customary that a contact-hole 4 is formed through an interlevel insulator layer 3 in order to electrically contact an electrode or upper layer interconnection (hereinafter simply referred to as an upper layer interconnection) to a semiconductor region formed on a semiconductor substrate 1 or lower layer interconnection 2 formed on the semiconductor substrate 1.
In this case, in order to form the upper layer interconnection into the contact-hole 4 with a satisfactory coverage without cavities or the like that causes a contact resistance, as shown in FIG. 1B, it is not preferable that the contact-hole 4 is shaped as a barrel which is narrow in the inside in cross-section.
Therefore, in order to form the above-mentioned contact-hole 4, an etching process with excellent anisotropy must be carried out through an opening 5a of an etching mask 5 that is formed on the interlevel insulator layer 3 by a photoresist.
As a method of carrying out the anisotropy etching, there is known a reactive ion etching (hereinafter simply referred to as an RIE) in which charged particles (ions) are generated and an anisotropy direction is determined by the application of a bias voltage.
A parallel plate system RIE which carries out the RIE is carried out under a relatively high process pressure ranging from 10 to 100 Pa, i.e., under low vacuum degree. As a consequence, ions are considerably scattered when the process pressure collides with a gas seed of residual gas with the result that anisotropy in the etching process is checked by the scattering of ions as schematically shown in FIG. 1A.
In order to improve direction, i.e., anisotropy of ions, the process pressure must be lowered (high vacuum degree) or ions must be accelerated in the direction perpendicular to the surface of the substance with a larger energy, e.g., about 500 eV.
However, to increase an etching rate to some extent, a high density plasma is required, which imposes a restriction upon reducing a process pressure. Therefore, when ions are accelerated with a large energy, there is then the problem that ion radiation surface is damaged.
On the other hand, as shown in FIG. 2, in a magnetron RIE which carries out a high frequency discharge in a magnetic field, a plasma 9 is formed by a high frequency discharge, and an etched material 11 which should be etched is disposed on a cathode 10 to which a high frequency electric power from a high frequency power supply 7 is applied. When an ion 8 accelerated by an electric field of an ion sheath region 14 formed on the etched material 11 collides with the etched material 11, the ion seed loses an energy and discharges a secondary electron. This secondary electron has a negative charge and is therefore accelerated in the opposite direction to that of ion. The secondary electron is affected by a magnetic field and moved in a drift fashion of electric field E.times.magnetic flux density B so that, as schematically shown by an arrow a in FIG. 2, this secondary electron moves in a cycloidal fashion to scan the whole surface of the etched material 11. Thus, there is increased the probability that the electron and the gas seed collide with each other, thereby a higher-density plasma being generated at a lower pressure.
However, in this case, since the cathode area is finite, the drift of electron is ended at the end face of an electrode ID whose electric field is small wherein electrons are collected to generate a high density plasma portion 14, thereby making plasma irregular.
Irregularity of plasma deteriorates etching characteristics such as s problem of charge-up and irregular etching. Also, a voltage on the ion sheath region is indirectly determined by density, pressure, electric power or the like of the process and cannot be controlled directly. There is then the problem that the etching cannot be controlled accurately without difficulty.
Further, as another conventional plasma etching apparatus, there is known an etching apparatus based on an electron cyclotron resonance (hereinafter simply referred to as an ECR) system shown in FIG. 3.
In this ECR etching apparatus, an etched material 11 is disposed on a cathode 18 to which an electric power of low frequency of 100 kHz is supplied from a low frequency power supply 17.
Owing to an interaction between a magnetic field (875 G (Gauss) generated by a magnetic coil 15 and a microwave (2.45 GHz) supplied from a waveguide (not shown), as shown in FIG. 4, there is formed an ECR area 16 in which an electron e circles so as to wound around a magnetic field (magnetic flux B) generated by the magnetic coil 15. More specifically, the ECR is generated by making one cycle of this electron e and one cycle of the microwave coincident with each other. As a consequence, the microwave is absorbed by the plasma efficiently and there is increased the probability that an electrolytic dissociation occurs due to electron impulse.
Ions thus generated are pulled onto the etched material 11 on the cathode 18.
The proposed conventional ECR etching apparatus is disclosed in J. Vac. Sci. Technol. B3 (4) P1025 (1985) by Keizo Suzuki et al. This ECR etching apparatus achieved a low gas pressure (0.04.about.0.4 Pa) and a high plasma density (1.times.10.sup.11 .about.1.times.10.sup.12 electrons/cm.sup.3).
In this ECR etching apparatus, however, the ECR thereof is produced with a microwave of 2.45 GHz and in a magnetic field of 875 G. Therefore, this ECR etching apparatus needs a magnet which generates a high magnetic flux density, which unavoidably makes the apparatus large in size and expensive.
Further, in this conventional ECR etching apparatus, electrons collide with the wall surface and are lost so that a plasma density near the wall surface and a plasma density at the central portion of the ECR area 16 become different from each other. There is then the problem that the plasma becomes irregular.
Furthermore, in this ECR etching apparatus, as shown in FIG. 3 by arrows B which show the state that magnetic fluxes are generated, magnetic fields are curved near the portion in which the etched material 11 is disposed, thereby the shape processed by the etching process being deteriorated and also uniformity of the etching being deteriorated.
Japanese laid-open patent publication No. 3-68773 (corresponding to U.S. Pat. No. 4,990,229) describes a plasma processing apparatus of a helicon system. As shown in the above-mentioned related art, this plasma processing apparatus can generate a plasma at a low pressure (10.sup.-2 .about.10.sup.-1 Pa) with high density (10.sup.12 to 10.sup.13 electrons/cm.sup.2). This plasma makes effective use of Landau damping in which energy can be transmitted to electrons efficiently when a phase velocity V.sub.0 of drift wave in the plasma and a frequency that excites a plasma become coincident with each other.
The phase velocity of this drift wave is expressed as: ##EQU1## where k is the Boltzmann's constant, ne is the plasma density, Te is the electron temperature, B is the magnetic flux density and dn/dx is the plasma density gradient.
Conditions with which this high density plasma is generated are dependent on parameters which are difficult to be controlled, such as the plasma density, the electron temperature or the like, and are very difficult to be controlled. The plasma processing apparatus of this system also encounters with the problem that the plasma density does not become completely uniform in the diametrical direction.